Change of Phase

Change of Phase : When a substance goes from one form to another, (almost) always in the same order. solid liquid gas This goes for all substances, remember. Now, some terms. HEAT - HEAT

Change of Phase (for water) : solid liquid gas MELTINGBOILING FREEZINGCONDENSING 0ºC100ºC melting/freezing point boiling/condensation point (ice) (steam)(water)

3 Types of Heat Transfer When we say “heat transfer”, we mean, of course, when heat travels from one place to another (the Q in lab). There are 3 types: conduction, convection, and radiation. 1) Conduction: heat transfer that occurs by means of particles in a material, mainly electrons. Conduction occurs by electrons colliding and atoms colliding inside the object being heated. The electrons in the metal atoms and the atoms themselves are transferring the heat down the rod.

Let’s talk terms for a second: An insulator is a substance that does not conduct heat well. examples of good insulators: air (blankets), glass, snow examples of bad insulators:metals A conductor is a substance that does conduct heat well. examples of good conductors: metals examples of bad conductors:air, glass, snow Important point: good conductors = bad insulators bad conductors = good insulators

Look at the following picture, and tell me what you notice about good and bad insulation:

3 Types of Heat Transfer (cont’d) (BTW: “Fluid” = gas or liquid) 2) Convection: transfer of heat by motion of a fluid through currents. As a fluid is heated from below, the molecules at the bottom begin heating faster. They spread apart and become less dense. They are then buoyed upward, only to be replaced by cooler, denser fluid. Some examples:

3 Types of Heat Transfer (cont’d) 2) Convection(cont’d): the beach is an excellent illustration of the concept of convection currents. During the day, you get wind towards the beach. During the night, you get wind away from the beach. 65º 75º 50º

3 Types of Heat Transfer (cont’d) 3) Radiation: energy transferred by radiant energy in the form of electromagnectic waves. Here is the electromagnetic spectrum. Let’s discuss: The electromagnetic spectrum describes all forms of radiant energy. The wavelength, or distance from one crest to the next, determines the energy and type of wave.

3 Types of Heat Transfer (cont’d) 3) Radiation (cont’d): we are most familiar with visible light because our eyes are designed to detect it. We cannot see x-rays, for example, because our eyes are insensitive to it. short wavelength high energy long wavelength low energy RYOBGVI

Emission of Radiant Energy All substances at any temperature above absolute zero (-273ºC) emit radiant energy. The average frequency ƒ of the radiant energy is directly proportional to the absolute temperature T of the emitter: ƒ ~ T The sun has a high temperature and therefore emits radiation at a high frequency- high enough on the electromagnetic scale to be visible light. High frequency ƒ = many types of radiation, including visible light

Emission of Radiant Energy The earth, in comparison, is relatively cool. So the radiant energy it emits has a frequency lower than that of visible light. The radiation emitted by the earth is in the form of infrared or heat. Low frequency ƒ = infrared radiation, aka heat

Evaporation = A Cooling Process Liquid water changes to the gaseous phase by the process of evaporation. The molecules in a liquid move randomly and at a wide variety of speeds all the while bumping into one another and transferring KE. Some gain enough to break free of the liquid state and become water vapor, the gaseous form of water. So when a water molecule leaves a liquid, it takes some of the KE, i.e., heat from the sample, thereby cooling the sample. A change of phase from liquid to gas

So why do we sweat? To remove heat from our bodies. Evaporation is a cooling process, drawing heat as the water goes into gaseous form. Just like stepping out of the shower on a cold day, sweating regulates body temperature by getting rid of excess heat. Dogs, on the other hand, do not have sweat glands and must get rid of heat through panting. Dogs have wide tongues, which provide a large surface area for evaporation, allowing them to cool off.

Condensation = a warming process When gas molecules near the surface of a liquid are attracted to the liquid, the strike the surface with increased KE, adding to the overall heat of that liquid. Let’s take two examples: 1) When a gaseous water molecule in warm, moist air strikes a cold soda can, it loses it’s energy and transfers the heat it has to the can of coke, warming the can and losing the heat it needs to remain a gas. This happens a few billion times, and the can starts to “sweat”, or collect liquid water from the air. Also, the can of coke gradually warms as these billions of molecules transfer their heat into it. A change of phase from gas to liquid

Condensation = a warming process 2) When you take a hot shower, the water vapor (steam) in the air makes evaporation difficult, and you feel much warmer inside the shower than outside. This is because the steam is condensing on your skin and transferring heat into your body. But this also explains why humidity during the summer is so miserable: your body cannot evaporate water from the skin and it is very hard to cool off. A change of phase from gas to liquid

Evaporation: Salt Ponds Loose outer electrons Liquids and gases?

Evaporation: Lake Powell

Condensation

Condensation: Atmosphere

Saturation –The faster molecules move the less able they are to condense Relative Humidity –Higher temp = more vapor needed to saturate

Relative humidity--the measure of how much water vapor is held in the air compared to how much could be held (at a given temperature)

Boiling = evaporation within a liquid The bubbles that form in a boiling liquid are not air, but the liquid becoming a gas within the liquid. So in boiling water, steam forms bubbles under water and then rises to the top to escape into the atmosphere. Now, the reason that water cannot become hotter Than 100ºC is because that at that point, any heat you add to it is used to evaporate water, which, as you know, is a cooling process. Therefore, the water is constantly cooled by the steam that is leaving from the surface. So the more you heat it, the more the water evaporates, cooling the water.

Energy is needed for change of phase! solid liquid gas MELTINGBOILING FREEZINGCONDENSING (ice) (steam)(water) Energy is absorbed when change of phase is in this direction Energy is released when change of phase is in this direction

Energy is needed for change of phase! 1) The amount of energy needed to change any substance from solid to liquid is called the heat of fusion. * The heat of fusion for water is: 335 joules/gram 2) The amount of energy needed to change any substance from liquid to gas is called the heat of vaporization. * The heat of vaporization for water is: 2255 joules/gram solid liquid gas MELTINGBOILING FREEZINGCONDENSING (ice) (steam)(water)

Energy is needed for change of phase! solid liquid gas MELTINGBOILING FREEZING CONDENSING (1 g ice) (1 g steam)(1 g water) 335 j / 80 calories2255 j / 540 calories 335 j / 80 calories2255 j / 540 calories Requires Energy Gives Off Energy

Freezing

Regelation

Energy and Change of Phase